1. Field of the Invention
This invention relates to methods for removing contaminants by filtration from chemical process streams, such as feed streams to a chemical reactor.
2. Technical Background
Filtration of the feedstock of a chemical reactor continues to be an area of ongoing research and need. In the petroleum refining industry, significant factors are driving the need for more feedstock filtration. Examples of such factors include: changes in raw material characteristics, evolving refinery productivity goals and tighter environmental laws and regulations. Further, refiners are upgrading a greater percentage of each barrel into higher value fuel products, but at the same time they must reduce the sulfur content of fuel products. This upgrading process commonly involves the use of fixed bed catalytic reactors. Because of the high costs of the catalysts, refiners want to maximize the flow through and overall life of the fixed bed catalysts. Unfortunately fixed bed catalytic reactors are subject to reduced flow and eventual plugging because of solids, carbon deposits or large molecular weight organic materials that become deposited on or lodged within the catalysts. When this happens the catalyst must be skimmed and/or completely replaced. Feedstock filtration is a preventative action to circumvent premature catalyst bed plugging and pressure drop development.
Another area of long felt need is in the ability to filter fluids (including gases) in between stages in a multistage process. In particular, effective filtration at a high throughput with minimal pressure drop has long been a desired goal.
Attempts to achieve the above desired goals have included the use of a variety of screens, including slotted-wedge wire screens, woven wire mesh, support ceramic materials, and multilayered, sintered media. It should be appreciated that each of these methods ultimately relies upon cake filtration, in which a filter cake is accumulated over time along with increasing back pressure and decreasing flow through. Alternatively the literature reports the use of reticulated ceramic as a filtration media. Reticulated ceramic is a multi-cellular sponge-like solid material that does not have discrete fluid flow pathways. During the course of the useful life of the reticulated ceramic, the pores and fluid flow pathways become clogged and the material rapidly reverts to being a cake filtration element with high back pressure, reduced flow rate and increasing pressure drop.
Fixed-shape bed-grading and bed-topping catalysts have been commercially available since at least 1987, when Haldor Topsoe Inc. first introduced “TK-10”, a press-formed cylindrical “wagon wheel” ceramic/alumina particle with 7 round holes. Other manufacturers have followed with similar products, such as Criterion 855MD (19 mm round medallion with 1 round and 4 elliptical holes), Akzo Nobel KG-50 and KG-55 (19 mm disk with five triangular channels), Catalyst Trading Co., Ltd.'s BT-500, Crystaphase BG-2000, UNICAT AOS-7H, and UNICAT UDC-1000. Nevertheless, for the reasons described above, there remains and exists an unmet need for methods of filtering fluids with high throughput, low pressure drop, and improved accuracy, especially in chemical reactors. The present disclosure describes a number of methods that fill this unmet need.